1. Field of the Invention
The present invention relates to an optical sheet laminating method, an optical sheet laminating device, a display device, and the like for laminating an optical sheet on a display panel.
2. Description of the Related Art
In accordance with demands for highly sophisticated functions of recent display devices, a unique display device capable of stereoscopic images, viewing angle controls, and the like through laminating an optical sheet such as a lenticular lens sheet, a prism sheet, or a diffusion sheet on a display panel that uses electro-optical elements such as liquid crystal has come to be used.
As an example of such display device, a display device using a lenticular lens sheet will be described. FIG. 21A is a perspective view showing a lenticular lens sheet, and FIG. 21B is a schematic view showing a stereoscopic display device using the lenticular lens sheet.
As shown in FIG. 21A, a lenticular lens sheet 110 has a flat face on one of the surfaces, and a plurality of cylindrical lenses 111 each having columnar surface and a semicircular (roughly a segment shape) cross-sectional shape are repeatedly provided in parallel on the other surface.
As shown in FIG. 21B, a left-eye pixel 115a and a right-eye pixel 115b are disposed alternately on a display panel 114 by corresponding to focal points of each of the cylindrical lenses 111. When the left-eye pixels 115a and the right-eye pixels 115b are driven by a drive circuit, not shown, according to prescribed signals, a left-eye image is formed in a left-eye region 120a and a right-eye image is formed in a right-eye region 120b, respectively, by the cylindrical lenses 111, so that an observer can recognize a stereoscopic image. It is needless to mention that a normal two-dimensional image can also be displayed through driving the left-eye pixels 115a and the right-eye pixels 115b by a same signal.
Further, as a display device using a lenticular lens sheet, there is a plural-image simultaneous display device which displays a plurality of images simultaneously. This uses the same method as that of the stereoscopic display, with which different images can be displayed to a plurality of observers by distributing images to the observing directions by the cylindrical lenses.
For such display devices using a micro lens array and a lenticular lens sheet, it is required to mount the lenticular lens sheet and the like on the display panel with high accuracy in order to acquire a high-quality stereoscopic image display or a plural-image simultaneous display. Particularly, for a high-definition display device loaded to a recent terminal device and the like, it is required to achieve lamination with accuracy of higher than ever before, which is lamination accuracy in an order of μm.
In order to laminate the optical sheet such as a lenticular lens sheet on the display panel with high accuracy, it is necessary to provide position alignment marks on each of the optical sheet and the display panel, and laminate those through executing an alignment action by reading those marks. This technique will be referred to as “related technique 1” hereinafter.
In the related technique 1, it is necessary to form each marks on the optical sheet and the display panel in an order of μm for achieving the lamination accuracy in an order of μm. For example, the distance of the mark on the lenticular lens sheet from the vertex of the cylindrical lens is required to be accurate in an order of μm. However, in general, it is difficult to form the mark accurately in an order of μm at the time of manufacturing the optical sheet with a machine work. In the meantime, another lens mark reading method is disclosed in Japanese Unexamined Patent Publication 2009-223193 (Patent Document 1: see FIG. 3 and FIG. 8). Hereinafter, this technique is referred to as “related technique 2”. In the related technique 2, no special mark is formed on a lenticular lens sheet, but light is irradiated onto the lenticular lens sheet and positional information of the cylindrical lenses is read from a transmission light luminance distribution generated according to a lens image forming performance. For the display panel, panel marks are captured via the cylindrical lenses, and the positions are aligned based thereupon.
Further, Japanese Unexamined Patent Publication 2009-222903 (Patent Document 2: see FIG. 39) discloses steps for laminating an optical element array sheet on a display panel by using a curved-type optical element holding head.
However, there are followings issues with the related technique 1. An optical sheet mark is placed on the surface of the optical sheet, and a panel mark is placed on the surface of the display panel. For example, in a case where the marks on both are superimposed with each other, an image thereof is captured by a camera, and each mark is read from the image, it is difficult to align the focal point simultaneously with both marks since the distances to both marks from the camera are different. This causes a problem in reading the marks.
Referring to a case of a liquid crystal display device as a way of example, as shown in FIG. 22A, a panel mark 132 is formed on a drive substrate 152 or a counter substrate 153, and an optical sheet mark 150 is formed on an optical sheet 151. Therefore, it is necessary to align the focal point separately with the optical sheet mark 150 and the panel mark 132 when capturing the images thereof by a same camera since there are the counter substrate 153, a polarization plate 154, and the optical sheet 151 existing therebetween. That is, the reading accuracy of both marks depends on the feeding accuracy of the focal point direction of the camera. Further, since it takes more time for aligning the focal point, it is disadvantageous in terms of tactics. Further, because of the structure to read the panel mark 132 via the optical sheet 151, the position of the panel mark 132 is observed by being changed due to a refractive effect of the optical sheet 151. Therefore, it is necessary to perform correction thereof. Further, the luminance distribution of transmission light acquired by irradiating light onto the optical sheet 151 largely depends on the image forming performance of the cylindrical lens. However, in a case where variations in the radius curvatures of each of the lenses are great or in a case where there is distortion generated in the optical sheet 151 itself, for example, the luminance distribution changes nonuniformly within the surface. This leads to deterioration of the mark reading accuracy.
In addition, for superimposing the optical sheet mark 150 with the panel mark 132 on one another, the panel mark 132 is disposed directly under the optical sheet 151. For example, with the liquid crystal display device, as shown in FIG. 22B, the external shape of the optical sheet 151 is smaller than the external shape of the display panel 131 and the external shape of the polarization plate 154 by one size. Thus, “the panel mark 132 comes directly under the optical sheet 151” means that the panel mark 132 is disposed near a display region 155 of the display panel 131. With a normally-white liquid crystal display device in particular, the panel mark 132 which causes leak of light (shield of light) is formed near the display region 155. Thus, there is a great influence imposed upon the display quality.
In addition to those issues of the related technique 1, a new issue has been found as a result of studies done by the inventors of the present invention regarding steps for laminating a highly accurate and highly reliable lens and a display panel. For example, with a lenticular lens sheet as one of the optical lens sheets, there may be a case where the lens pitch becomes nonuniform within the surface as shown in FIG. 23 mainly due to the manufacture process of the lenticular lens sheet. For example, there are various patterns of cases where the lens pitch becomes nonuniform, e.g., a case where the lens pitch becomes larger towards the upper side as in FIG. 23A, a case where the lens pitch becomes larger towards the center as in FIG. 23B, and a case where the lens pitch becomes smaller towards the center as in FIG. 23C. Such nonuniformity of the lens pitch greatly affects the visually recognizable distance where the stereoscopic viewing field becomes the maximum and the size itself of the stereoscopic viewing field in a stereoscopic display device. Therefore, it is necessary to ease the influence of the lens pitch fluctuation when laminating the optical sheet.
Further, when holding an optical element forming surface of the optical sheet by using a sheet holding head, the substantial contact area between the optical sheet and the sheet holding head becomes small since the optical element forming surface has fine protrusions and concaves. For this reason, there is such an issue that the force for holding the optical sheet is reduced.
In the meantime, while the related technique 1 reads the positional information of the optical sheet mark by using existence of light transmitting through the optical sheet, the related technique 2 reads the positional information of the lens by using the luminance distribution of light transmitting through the optical sheet. That is, the related technique 2 acquires the positional information by using the light transmitting through the optical sheet as in the case of the related technique 1, so that it faces the same issues. Further, the sheet holding head (a holding frame that holds the lenticular lens sheet) of the related technique 2 is formed with a material that exhibits light transmitting property for not disturbing imaging done by an imaging part (see paragraph 0022 of Patent Document 2). Therefore, the materials used for the sheet holding head of the related technique 2 are limited to fragile glass, plastics, and the like, so that solid metals, ceramics, and the like cannot be used.
The present invention is designed to overcome such issues. An exemplary object of the present invention is to provide an optical sheet laminating method and a laminating device using the method which can mount an optical sheet on a display panel with high yield and high accuracy as well as to provide a high-quality display device manufactured by using the laminating method.